A new instance of the handler registered under the key, or null. Uses GetRegistration rather than CreateInstance
because CreateInstance silently falls back to the (abstract) base class for unknown keys. ClassFactory keys match
trimmed and case-insensitively; every call returns a fresh instance because handlers may keep per-delivery state.
A new instance of the handler registered under the key, or null. Uses GetRegistration rather than CreateInstance because CreateInstance silently falls back to the (abstract) base class for unknown keys. ClassFactory keys match trimmed and case-insensitively; every call returns a fresh instance because handlers may keep per-delivery state.